Method and means for combining and for eliminating frequencies



Jan. 20, 1931. c. w. HA'NSELL 1,739,354

METHOD AND MEANS FOR COMBINING AND FOR ELIMINATING FREQUENCIES med Nov. 28, 1925 2 sums-Sam 1 P1475 AMPS.

CLARENCE W. HIRNSELL ORNEY Jan. 20, 1931. c. w. HANSELL 1,739,354

METHOD AND MEANS FOR COMBINING AND FOR- ELIIINATING FREQUENCIES Filed No v. 28., 1925 2 Sheets-Sheet 2 l N VENTOR CLARENCE W. HANSEN.

TORNEY Patented Jan. 20, 1931 UNITED STATES PATENT OFFICE CIIABENCE W. EANSELL, OF ROCKY POINT, NEW YORK, ASSIGNOB T0 RADIO CORPORA- TION OF AMERICA, A CORPORATION OF DELAWARE METHOD AND MEANS FOR COMBINING AND FOR ELIMINATING FREQUENCIES Application filed November as, 1925. Serial m. 71,890.-

The present invention relates to an improved circuit for the control of one wave by another for signalling or other purposes. Circuits and arrangements of this general type commonly known in the art are modulators, and demodulators, according to the use to which they are put.

The invention relates particularly to a novel form of balanced modulator or detector in which preferably a pair of three elect-rode discharge devices are connected so as to emphasize the production of certain wave components and to neutralize or lessen the production of other components.

In the constant current or Heising system for modulation, similar to that shown in Patent No. 1,442,147, the antenna current cannot be modulated to zero and twice the carrier amplitude without distortion due to change in curvature of the characteristic curve of modulator tubes at its ends; that is, at one extreme the output cannot be reduced to zero because the modulator tubes never become a complete short circuit, while in the other extreme they do not become a complete open circuit. If either extreme is even approached there is distortion. The

' ordinary type of modulator which functions by virtue of the curved characteristic of a vacuum tube has the same limitations. In order to overcome the distortion in these types of modulators, it is proposed to use a circuit with two tubes arranged in a manner similar to that used for suppressing the carrier for side band transmission. This circuit is illustrated by Fig. 1 of the drawings. In this arrangement the master oscillator supplies equal oscillation to two vacuum tubes. \Vhen these tubes have equal direct current grid bias, they give equal outputs and these outputs are balanced in the output transformer so that the resulting output of the complete circuit is zero.

A microphone circuit .acting through a suitable transformer is used to produce a low frequency voltage which will change the grid bias potentials differentially so as to unbalance the output from the to es and result in anoutput from the system. If this is done the resulting output contains only the two side bands of the modulated carrier. This represents the ordinary push-pull modulating system shown in the patent to Hartley 1,494,905.

If then the tubes are given a fixed unbalance by B and C batteries, the carrier is transmitted as well as the side bands. Then, when the .microphone is used to modulate the output, the result is identical with that of the ordinary modulating system, with the exception that the modulation can be carried completely down to zero and up to twice the carrier amplitude. In addition, the A, B and C batteries can be so adjusted that the modulation characteristic is a straight line passing through zero so that there is no distortion. We can therefore obtain practically distortionless modulation of 100%.

It is interesting to note what will happen if the modulation is carried below zero. In such a case the modulation will exceed 100%, where modulation is defined as the ratio of the maximum change away from the normal carrier current, produced by the side band currents, to the steady unmodulated carrier current. With 100 ampere carrier current in an antenna and 100% modulation the antenna current will vary from 0 to 200 amperes. With 200% modulation the antenna current will vary from 100 amperes to 300 amperes, the significance of the minus sign being a 180 reversal in phase. Modulation in excess of 100% is analogous to a dearth of carrier frequency energy, and consequently distortion will be produced in an ordinary receiver. However, if the carrier is amplified at the receiver to a greater extent than the side bands, it is possible to correct the distortion again. With modern filters andmethods, the separation of the carrier at the receiver is not extremely difficult over the whole range of carrier frequencies used in practice, although it may be necessary, for very high carrier frequencies, to use a double detection or heterqdyne receiver with filters adjusted to operate upon the energy of intermediate frequency in order to make the carrier separation less difficult.

It therefore is feasible to use, as well as possible to construct a transmitter in which the differential grid potential variations are larger than the amount of unbalance, and so produce more than 100% modulation. Also there is on discontinuity in the modulation near 100%, as there is in all other systems of modulation now in use. The possibility of obtaining more than 100% modulation is useful in that it makes it possible to suppress one side band and still have 100% modulation b making the total modulation due to both slde bands approach 200%. Using the figures before employed, if the steady antenna current is 100 amperes, and if the grid unbalance is such that both side band currents have sufficient magnitude to decrease the antenna current to 100 ampe'res or to raise it to 300 amperes, then if one side band is suppressed the remaining side band will decrease the carrier current to 0 and raise it to 200 amperes, which is 100% modulation on single side band and carrier transmission.

It is also possible to construct a radio communication system having the characteristics and advantages of a modulated carrier, yet having much of the carrier added at the receiver, thereby attaining power economy and reducing the rating and costs of the antenna and power equipment.

The use of the balance principle makes it unncessary to modulate the tubes over the whole range of their characteristics from twice the steady value down to dynamic cutoff and so avoids the discontinuities and distortion usually caused as soon as the peak of the high frequency excitation and the modulation reaches dynamic cut-off, at which time arts of the excitation and modulation are a ruptly cut-01f, and distortion suddenly increases rapidly.

The amplification characteristic of a great many ordinary vacuum tubes as actually operated follows approximately a threehalves power law, and the rate of change of the slope of the amplification characteristic, which expresses the modulation characteristic, then follows a squareroot power, when the output impedance is small. The modu lation characteristic of a single tube is therefore curved and as a result the modulation in the output, for large amplitudes of modulation, will be distorted by the addition of a very large second harmonic frequency. The use of the two balanced tubes balances out this second harmonic, giving a resulting modulation characteristic which is more nearly straight. This correction of the distortion is the most important characteristic of the balanced modulator.

The modulator described is perfectly simple in its analysis and in its applications and ad'ustments, which is not so much true of the eising system. The latter system depends for its satisfactory operation upon rather criticaladjustments in order to keep the oscillator going when the direct current plate voltage is reduced to a minimum by the modulation. It also should preferably use a grid leak in order to get an automatic adjustment of the grid bias potential to correspond to the late potential, but in actual practice it is 0 en not practicable to use a grid leak bias because of the uncertain direction of the grid current of the tube. The Heising system is also not capable of 100% modulation because the impedance of the modulator tubes can not be reduced to zero. It is subject to distortion if 100% modulation is even approached.

The modulator of this invention is of the same mechanical construction as the balanced modulator units which have been developed for use in sin le side band and multiplex equipments, and its use is a simple but radically different application of standard equipment The vintra-inter-stage push-pull circuit shown in Fig. 3 is an improvement over the simple modulator.- Although the circuit shown in Fig. 1 eliminates even harmonics of the modulation it does not eliminate the fundamental and odd harmonics of the modulation nor the even harmonics of the radio frequency output caused by the curvature of the tube characteristics. I obviate these defects by using two complete sets of pushpull modulators which themselves are arranged to cooperate in push-pull. The circuit consists of two balanced modulator units so arranged that their desired outputs add in a common output circuit but the second and other even multiple harmonic radio frequencies are balanced out. This circuit is of considerable importance in a multiplex transmitter where an extremely large band of frequencies is required because of the waste frequency spectrum which would otherwise be taken up by the harmonics.

This system also will produce an appreciable reduction in the radiation of undesired and interfering harmonic frequencies from transmitters by preventing the formation of the harmonics before they can enter the amplifier system.

Further and more definite objects will be appreciated in connection with the follow-' ing specification, claims and attached drawings, in which Fig. 1 shows a simple modification of my arrangement.

ment.

Fig. 3 shows connections whereby undesirable harmonics are eliminated;

Fig. 4: shows the relation between the characteristic curves of an ordinary-balanced tube amplifier, the separate tube characteristics and the modulation characteristics of the amplifier described in this invention.

1 and 2 show respectively, in Figure 1, the

Fig. 2 shows a slightly different arrangerimary winding 3 windings 4 and 5. The oscillator 9 is coupled through the transformer 8 to the rid cirunits of the tubes 10 and 11. The Eias batteries 6 and 7 are disposed in opposite directions so that opposite potentials are impressed upon the grids of the tubes.

The outputs of these tubes are also balanced through the primary coils 13 and 14 being 'potential source indicated at '12.

connected at their middle point by the high These coils are coupled to the output circuit through winding 15. It can be seen that equal and opposite potentials impressed upon coils 13 and 14 will produce no result in the winding 15 and that in order to have any potential effect upon the terminals of coil 15 it will be necessary to produce results in the coils 13 and 14 in similar directions.

In Fig. 2 the bias batteries 6 and 7 of Fig. 1 are replaced by a common battery .16 and the different potentials which are applied to the grids are determined by variable potentiometers 17 and 18 which are used to pick-off the desired potential. This battery 16 is grounded at 19 and completes itscircuit through the ground 20 back to the-potentiometers. This also forms a convenient method of grounding the filaments of the tubes.

21 is the oscillator for producing radio frequency oscillations in the system which are by-passed through the condensers 23 and 24 to produce oscillations in the output coils 25 and 26. The central point of these coils is connected to the high potential battery 27 which completes thecircuit to the ground 28 and back to the filaments through ground 20. The operation of this arrangement is theoretically the same as in Fig. 1.

The two arrangements thus far described are subject to inconvenience in operation on account of the possibility that some harmonics of the carrier wave will be transmitted to the antenna circuit. In order to neutralize many of these harmonics, an intra-inter-stage pushpull arrangement has been devised as in Fig.

3. Here the input coils are connected at 29 and 30 coupled to two split secondary transformers 31, 32 and 33, 34. These secondaries excite the grids of tubes 42, 43 and 45, 46, respectively, each half coil exciting its respective grid independently.

As in the case of Fig. 2, thefilaments are grounded at 53; ground 36 is connected to the bias battery and current is conducted from this battery to the potentiometers 37 and 38 back to the ground by means of wire 57. In this case, potentiometers 37 and 38 are used to pick-off the respective bias voltages for the pairs of tubes 42, and 43, 46 respectively,

the grids of the tubes in the pairs being connected by wires 40 and 39 so that tubes of one pair will have a. different bias than the tubes of the other pair and the tubes of the upper push pull group and of the lower push pull group are differently biased by equal amounts. The output coils 49 and 50 of the tubes are connected in opposition as before with the wire 54 connecting their middle points to the high potential battery 51 and back through the ground 52.

In addition to the modulating input produced in coils 29 and 30 a master oscillator 41 is used, capacitatively coupled to the grids of the tubes through the inductive coupling 48 and the capacitive couplings 44 and 47.

It can be seen from the arrangement as shown the two sets of tubes will be energized by this high frequency equally and capacitatively so that any total unbalance between the four halves of the output coils will be manifest in the secondaries 55 and 56 and that the modulating frequency and all of its harmonies as well as all harmonics which are even multiples of the radio frequency voltages will be neutralized. An unbalance may be caused by the effect of current in the modulating coils 29 and 30 which acting through their couplings to coils 31, 32 and 33, 34 affect the grids of the tubes to cause resultant unbalance current to flow in opposite directions in the output coils 49 and 50. This impresses a voltage on the secondary coils 55 and 56 which being connected in opposite directions will produce the desired effect upon any apparatus connected to the terminals.

Referring to Figure 3, the upper two tubes form one push-pull group and the lower two tubes another, and because of a fundamental property of a push-pull circuit, in each group the even harmonics of the modulating frequency are suppressed, and cannot appear in the output circuit, 55, 56. The fundamental and the odd harmonics of the modulating frequency do appear in the coils 49 and 50, but are in phase, and therefore are eliminated by the reversed connection of the windings 55 and 56.

It is clear from the figure that the carrier current from the source 41 is supplied to each push-pull group symmetrically, and, therefore, in each push-pull group, if the grids were equally biased, the carrier would be suppressed and could not appear in the coils 49 and 50. The grids in each group are not at equal bias,and therefore a part of the carrier dependent upon the amount of inequality of bias appears in each of the coils 49. and 50. The carrier is fed to the upper and lower groups in phase opposition, that is, when the carrier is positive at 44 it is negative at 47 In this way the carrier is supplied to the upper group and the lower group in pushpull, as'regards the pair of groups. For this reason the carrier components in the coils 49 and 50 are in phase opposition, but because of the reversed connection of coils 55 and 56,

Ill

the carrier components are combined additively and do appear in the output circuit.

The even harmonics of the carrier experience between coils 49 and 50 a phase reversal relative to the fundamental of the carrier which is most simply explained by the analogous case of the phase opposition of an even harmonic in a push-pull circuit. The fundamental or carrier being in phase opposition, it follows that the even harmonic frequencies are co-phasal, and since the reversed connection of the output circuit combines the components subtractively, the even harmonics are neutralized.

In addition due to the fact that there is a permanent difference of bias between the tubes of each pair, some of the carrier wave of an intensity proportionally equivalent to this grid bias will also be transmitted. In this way, a carrier wave can be produced which will be commensurate with the amount of modulation impressed by the microphone or other equivalent means. It can also be seen that with such a carrier wave no distortion of the modulation will result due to the curvature of the tube'characteristics near its end and that the full advantages of the balanced modulator are still retained without the disadvantages of necessitating the addition of a carrier in receiving any signals sent by this arrangement.

59 and 61 of Fig. 4 show the relative tube characteristics as they would be with their filaments connected together, the values above the abscissa being positive and those below being negative. Where the tubes are to be used as amplifiers and are so connected with the proper transformers a characteristic would result as at which is the numerical sum of the instantaneous value of the two curves 59 and 61. When these tubes are used as modulators, the modulation characteristic of each tube is a curve expressing the first derivative of the amplification characteristics 59 and 61, and adding these two modulation characteristic curves, a characteristic such as 58 results, and while working within the limits of the relatively straight portion of the curve, substantially straight line modulation is possible both in a positive and negative direction.

This disclosure is not intended to be limited to the use of two tubes or two sets of tubes but could be as well used with three or more which would result in different advantages in the elimination of harmonics and greater power.

I wish to be limited only by the scope of the appended claims.

I claim:

1. A method of reducing radio frequency harmonics in a carrier fundamental in order to make possible the use of more carrier frequencies in a given frequency spectrum which includes suitably directly balancing the outputs of two sets of simultaneously operated push-pull modulators by opposing them in phase, and supplying carrier. energy to the sets of modulators in phase opposition.

2. In combination, sets of vacuum tubes having cathodes, anodes and grids, each of said sets having their grids capacitatively coupled, their anodes inductively coupled and their cathodes conductively coupled, an oscillator coupled to each capacitative grid coupling, modulating means coupled to the grids, one grid of each set being conductively coupled to one grid of another set, means for conductively coupling the plates and applying a constant potential at the point of conductive coupling, a bias means for applying unlike potentials on each grid in a set and like potentials on all conductively coupled grids, means inductively associated with the inductive anode couplings so arranged as to counteract simultaneous equal oscillations and to be effective for simultaneous opposite oscillations.

3. In combination, sets of vacuum tubes having cathodes, anodes and grids, each of said sets having their grids capacitatively coupled, their anodes inductively coupled and their cathodes conductively coupled, an oscillator coupled to each capacitative grid coupling, modulating means coupled to the grids,one grid of each set being conductively coupled toone grid of another set, means for conductively coupling the anodes and applying a constant potential at the point of conductive coupling, a bias means for applying unlike potentials on each grid in a set and like potentials on all conductively coupled grids, means inductively associated with the inductive anode couplings so arranged as to counteract simultaneous equal oscillations.

4. The method of eliminating undesired frequencies when combining energies which includes subdividing one of the energies into four components, utilizing pairs of components in push pull, recombining the pairs in push pull to eliminate even harmonics, and combining the push pull outputs in phase opposition to eliminate the fundamental and odd harmonics.

5. The method of reducing harmonics produced by modulation of energy of one frequency by energy of another frequency which includes applying energy of one of the frequencies in push-pull to each of a pair of push-pull modulators, applying energy of the other frequency symmetrically to each modulator but in push-pull to the pair of energy to two pairs oftubes in phase and to each pair in push-pull, applying the carrier energy to the two airs o opposition and to eac pair symmetrically, differently normally biasing the tubes of each pair, and combming the outputs of each pair in push-pull, and of the two pairs in I phase opposition.

7. In combination, a plurality of pairs of electron emission tubes, means to supply energy to each pair in series and to the pairs cophasially, means to supply energy to each pair in parallel and to the pairs in phase opposition, and means to subtractively combine energy from the tubes.

8. In combination, a plurality of pairs of electron emission tubes, means to supply energy of one frequency to each pair in series and to the pairs cophasially, means to supply energy of another frequency to each pair in parallel and to the pans in phase opposition, and means to collect energy from each pair in series and from the pairs in phase opposition.

9. An arrangement for the elimination of undesired frequencies comprising, in combination, two push-pull stages, an input circuit coupled to the input circuits of the two stages cophasially, and an output circuit coupled to the output circuits of the two stages in phase opposition.

10. In combination, a first push-pull stage, a second push-pull stage, means to supply ener of a first frequency to each stage in push-pull and to both stages in hase,-means to supply energy of a second equency'to each stage symmetrically and to both stages in push-pull, and means to'collect energy from each stage in push-pull and from bot stages in phase opposition.

CLARENCE w. HANSELL.

tubes in phase 

